1. Field of the Invention
This invention relates to a pedal crank mechanism for a bicycle, and more particularly to an improvement of a bicycle equipped with a driving force input device having a swinging fulcrum which is disclosed in Japanese Patent Application Laid-Open No. H11-263275 (patent document 1).
2. Related Art
The pedal crank mechanism disclosed in the patent document 1 will be explained using the terms which are used in the description of the present invention. This mechanism comprises an active crank 24 supported through a first fixed fulcrum shaft 25 as a shaft of a driving wheel, a follower crank 21 supported on a frame in the vicinity of the driving wheel 26 through a second fixed fulcrum shaft 31, and a connecting lever 22 connected to a free end of the active crank 24 through a first free fulcrum shaft 35 and also connected to a free end of the follower crank 21 through a second free fulcrum shaft 32.
The mechanism further comprises a pedal 23 attached to an end part opposite to an end part on the second free fulcrum shaft 32 side of the connecting lever 22 and spaced apart from the first free fulcrum shaft 35.
The active crank 24 is caused to rotate about the first fixed fulcrum shaft 25 in unison with the driving wheel 26 by the stepping force exerted to the pedal 23, and the rotational motion of the active crank 24 is transmitted to the passive crank 21 through the connecting lever 22 so that the passive crank 21 is swung about the second fixed fulcrum shaft 31, thereby rotating the pedal 23 along a non-circular track (pedal track) T.
The pedal crank mechanism for a bicycle disclosed in the patent document 1 has the following constitutional features A′ through D′ in comparison with those of the present invention:                A′. the second fixed fulcrum shaft 31 is arranged on a horizontal line passing through the first fixed fulcrum shaft 25, i.e., a line connecting the shaft 1a of the front wheel 1 and the shaft 2a of the rear wheel 2 with each other;        B′. the passive crank 21 is extended upward beyond the line connecting the first and second fixed fulcrum shafts 25, 31 with each other from the second fixed fulcrum shaft 31 such that the second free fulcrum shaft 32 at its free end is swung above the line connecting the first and second fixed fulcrum shafts 25, 31 with each other;        C′. the connecting lever 22 has a bent configuration like a dogleg, the lever 22 is connected at its intermediate bent part to the free end of the active crank 24 through the first free fulcrum shaft 35, and the pedal 23 is attached to the connecting lever 22 part extending to the opposite side to the second free fulcrum shaft 32 from the first free fulcrum shaft 35 of the connecting lever 22; and        D′. the pedal 23 is rotated at a front part of the first fixed fulcrum shaft 25 of the active crank 24, the pedal 23 is rotated along a deformed prolate elliptical tack (pedal track) T which is long in the up and down directions at its front part but short in the left and right directions, and the first fixed fulcrum shaft 25 of the active crank 24 is located at an external area of the deformed prolate elliptical track T.        
It should be noted that the patent document 1 refers to the official gazette of Japanese Patent Application Laid-Open No. H11-263275.
As apparent from the deformed prolate elliptical track (pedal track) which is, as typically shown in its FIG. 5, long in the up and down directions and short in left and right directions, the pedal crank mechanism for a bicycle shown in the patent document 1 has such advantages that the pedal stepping force is, as a whole, increased in power factor. However, it has the following disadvantages. In the rotational track at the time of stepping the pedals toward the lower rotational dead point from the upper rotational dead point performed alternately by the right leg and the left leg in the related art, a stepping force chiefly in the vertical direction is required and this turns out to be the cause of fatigue to the rider.
In consideration of, and from the view point of, balancing the muscles of the entire leg, load is imposed on the local muscles for chiefly vertically moving the pedals and as a result, fatigue is increased. Thus, the bicycle equipped with this mechanism is not suited for a long distance riding tour.
Moreover, this mechanism of the related art tends to force the leg to form an unnatural bending shape in the returning track. This makes it difficult for the rider to keep riding the bicycle and the rider experiences an increased fatigue.